The invention relates to a trench process, and more particularly relates to a method for making a concave bottom oxide within a trench.
A conventional trench process usually begins with the defining of the hard mask overlying the semiconductor substrate to form an opening exposing the surface of the semiconductor substrate. Then, the exposed semiconductor substrate within the opening is removed by means of wet- or dry-etching to form a trench. Subsequently, a silicon dioxide is formed overlying the side-walls and the bottom of the trench by means of thermal oxidation to oxidize a portion of the semiconductor substrate. However, the edge and the bottom of the trench could be rounder, thereby avoiding weak points in the edge of the trench, which results in leakage current.
Since the method described above comprises a thermal oxidation, the edge of resultant trench usually has so called xe2x80x9cBird""s Beaksxe2x80x9d. Moreover, it is hard to well control the thickness of the silicon dioxide during the thermal oxidation process. Also, the leakage current will be apparent when the bottom oxide within the trench is too thick. Therefore, it is necessary to develop a novel method for making a concave bottom oxide within a trench to overcome the drawback of the above conventional trench method.
The process of the above conventional trench method is illustrated in detail in FIGS. 1Axcx9c1C and 2Axcx9c2C.
First, referring to FIG. 1A, a semiconductor substrate 100, such as a silicon substrate, is provided. Then, a pad oxide 110 (e.g. a silicon dioxide layer) and a hard mask layer 120 (e.g. a nitride layer) are formed on the semiconductor substrate 100 in sequence. Then, photolithography procedures and etching techniques are applied to define the nitride 120 and the pad oxide layer 110 to form an opening 130 exposing the surface of the semiconductor substrate 100.
Next, referring to FIG. 1B, using the hard mask layer 120 and the pad oxide layer 110 as a mask, the exposed semiconductor substrate 100 within the opening 130 is removed by means of wet-etching, thereby a V-trench 140 is formed.
Then, referring to FIG. 1C, a thermal oxidation process is applied to oxidize a portion of the exposed semiconductor substrate 100 of the V-trench 140, thus a trench 150 with an overlying silicon dioxide layer 110xe2x80x2 is formed.
Because the trench is defined by means of wet-etching, a V-trench is produced instead of a trench with a more vertical profile. In addition, the silicon dioxide layer 110xe2x80x2 is formed by means of thermal oxidation, thereby the xe2x80x9cBird""s beakxe2x80x9d is apparent in the edges of result trench 150.
First, referring to FIG. 2A, a semiconductor substrate 200, such as a silicon substrate, is provided. Then, a pad oxide 210 (e.g. a silicon dioxide layer) and a hard mask layer 220 (e.g. a nitride layer) are formed on the semiconductor substrate 200 in sequence. Then, photolithography procedures and etching techniques are applied to define the nitride 220 and the pad oxide layer 210 to form an opening 230 exposing the surface of the semiconductor substrate 200.
Next, referring to FIG. 2B, using the hard mask layer 220 and the pad oxide layer 210 as a mask, the exposed semiconductor substrate 200 within the opening 230 is removed by means of dry-etching, thereby a U-trench 240 is formed.
Next, referring to FIG. 2C, a thermal oxidation process is applied to oxidize a portion of the exposed semiconductor substrate 200 of the U-trench 240, thus a trench 250 with an overlying silicon dioxide layer 210xe2x80x2 is formed.
It is noted that the trench is defined by means of dry-etching, thereby a trench with a more vertical profile can be obtained. However, the silicon dioxide layer 210xe2x80x2 is still formed by means of thermal oxidation in order to produce a trench 250 with a rounder bottom, thereby the xe2x80x9cBird""s beakxe2x80x9d is still apparent in the edges of trench 250.
In order to address the drawback of the conventional trench process described above, it is necessary to develop a novel trench process to make a concave bottom oxide within a trench.
In order to address the drawback of the conventional trench process described above, this invention discloses a method for making a concave bottom oxide within a trench.
The feature of the invention is to provide a method for making a concave bottom oxide within a trench, the steps comprising: providing a semiconductor substrate; forming an insulating layer on the semiconductor substrate; defining the insulating layer to form an opening exposing the surface of the semiconductor substrate; dry-etching the exposed semiconductor substrate within the opening by using the first insulating layer as an etching mask to form a trench; depositing a first oxide layer conformably over the insulating layer, the side-walls and the bottom of the trench; depositing a second oxide layer on the first oxide layer and filling-up the trench surrounded by the first oxide layer; annealing to densify the first and second oxide layers; etching-back the first and second oxide layer to remove the portion overlying the first insulating layer, and forming a spacer consisting of the residual first oxide layer on the side-walls of the trench, and a concave bottom oxide consisting of the first and second oxide layers on the bottom of the trench.
In the method according to this invention described, the insulating layer comprises a pad oxide layer and a nitride layer. The nitride layer consists of silicon nitride or silicon oxynitride. The first oxide layer is formed by means of CVD using O3/TEOS as reactants, and the ratio of O3/TEOS is less than 16. The second oxide layer is formed by means of CVD using O3/TEOS as reactant, and the ratio of O3/TEOS is more than 16. The etching-back step is applied by means of wet-etching. The wet-etching step is performed by using 5% HF as an etchant.
Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.